1. Field of the Invention
The present invention is related to a process of forming a hydrophobic aerogel having excellent transparency, which is characterized by reacting a gel having a SiO.sub.2 skeleton structure with a hydrophobic agent under a supercritical condition of a medium contained in the gel.
2. Disclosure of the Prior Art
An aerogel is useful as a heat insulator having excellent transparency. As disclosed in U.S. Pat. Nos. 4,402,927, No.4,432,956 and No. 4,610,863, an aerogel is formed by the following process. That is, hydrolysis and condensation of an alkoxysilane are performed in the presence of a solvent to prepare a gel having a SiO.sub.2 skeleton structure. The alkoxysilane is also named as a silicon alkoxide or alkylsilicate. After the solvent contained in the gel is replaced with a medium, the medium is removed from the gel by a supercritical drying method to obtain the aerogel.
However, since the SiO.sub.2 skeleton structure of the aerogel includes silanol groups having hydrophilicity at a surface thereof, there is a problem that the silanol groups adsorb moisture in the atmosphere as the aerogel is used for many hours, so that optical and thermal properties of the aerogel are lowered, and also a distortion or shrinkage crack of the aerogel appears.
For improving the problem, International Application No. PCT/JP92/00156 discloses a process of forming a hydrophobic aerogel. In the process, after hydrolysis and condensation of an alkoxysilane are performed in the presence of a solvent to prepare a gel having a SiO.sub.2 skeleton structure, the gel is reacted with a hydrophobic agent to form a hydrophobic gel. The solvent contained in the hydrophobic gel is replaced with a medium, and then the medium is removed from the hydrophobic gel by a supercritical drying method to obtain the hydrophobic aerogel. However, since a penetration rate of hydrophobic agent into the gel is slow, there is a problem that it takes many hours for penetrating the hydrophobic agent throughout the SiO.sub.2 skeleton structure of the gel. In addition, there is another problem that transparency of the hydrophobic aerogel is sightly lowered as compared with transparency of the unreacted aerogel with the hydrophobic agent. It is believed that such a decrease of transparency of the hydrophobic aerogel is caused by insufficient penetration of the hydrophobic agent into the SiO.sub.2 skeleton structure of the gel.